Conventionally used solid absorbents in desulphurization processes are activated carbon, iron oxide and zinc oxide.
A solid acceptor comprising supported platinum group metals in association with copper, germanium, rhenium or tin compounds is mentioned in GB Patent No. 1,592,378. The acceptor as described is used in the separation of sulphur dioxide from a gaseous mixture containing sulphur dioxide and oxygen.
Sulphur in sulphur-containing fuel, however, is available in many processes as gaseous sulphides such as hydrogen sulphide, carbonyl sulphide and carbon disulphide or related compounds, such as mercaptans.
Removal of gaseous sulphide-compounds is usually achieved by cold scrubbing processes with liquid absorbents, such as alkanolamines.
The known scrubbing methods disadvantageously require cooling of treated gases, resulting in thermally less efficient processes.
Certain drawbacks of the known scrubbing processes can be avoided by the use of solid gas purification systems including the use of solid sulphide absorbents like metal oxides, which operate at higher temperatures.
A process for removing hydrogen sulphide from gases is mentioned in EP Application No. 159,05. By this process a hydrogen sulphide containing gas is contacted at a temperature above 300.degree. C. with a solid absorbent comprising a mixture of zinc oxide and a zeolite. Spent absorbent is regenerated by passing an oxygen-containing gas over the absorbent at a temperature above 400.degree. C.
Several other absorbents based on zinc oxide are described in the literature. Thus, zinc ferrite was recently proposed as absorbent of this type. Zinc ferrite (ZnO.Fe.sub.2 O.sub.3) is a regenerative absorbent which works at high temperatures. During the gas desulphurization step the zinc and iron oxides are converted into the corresponding sulphides, which can be regenerated to the oxides by using oxidizing gases such as oxygen or air at elevated temperatures.
According to the following reactions: EQU ZnS+3/2O.sub.2 .rarw..fwdarw.ZnO+SO.sub.2 (g); .DELTA.H=-105 kcal EQU ZnS+2O.sub.2 .rarw..fwdarw.ZnO+SO.sub.3 (g); .DELTA.H=-129 kcal
the regeneration of ZnS is a highly exothermic reaction resulting in sintering of the absorption mass at elevated temperatures and in decreased desulphurization capacity of the absorbent caused by structural changes. To minimize sintering a proper control of the reaction temperature and atmosphere is necessary to stabilize the temperature within narrow ranges during the regeneration step. Lower temperatures, however, lead to a significant degree of zinc sulphate formation via the following reactions: EQU ZnS+2O.sub.2 .fwdarw.ZnSO.sub.4 EQU ZnO+SO.sub.2 +1/2O.sub.2 .fwdarw.ZnSO.sub.4.
This is deleterious to the overall process in that the presence of sulphates not only reduces the sulphur capacity of the sorbent but also leads to the introduction of SO.sub.2 into the hot gas stream.
It is therefore an object of the present invention to provide a process for the purification of sulphide-containing gases without the disadvantages of the known processes.
According to the invention, there is provided a regenerative process for the removal of sulphides from a gas stream by using a solid absorbent comprising tin, tin oxides or mixtures thereof and optionally a stabilizing component, which process comprises the steps of contacting a sulphide-containing gas stream with the solid absorbent; sulphidizing the solid absorbent and desulphidizing the gas stream; and regenerating the sulphidized absorbent by contacting the sulphidized absorbent with a stream of steam.
In a preferred embodiment of the regenerative process according to the invention, the sulphide containing gas stream is contacted at a temperature of between 200.degree. C. and 600.degree. C. with the absorbent.
In a particularly preferred embodiment of the regenerative process according to the invention, the sulphidized absorbent is contacted at a temperature of between 300.degree. C. and 500.degree. C. with the stream of steam.
A further object of the present invention is to provide a regenerative absorbent for use in the process as described above, which absorbent comprises as absorption component at least one component selected from the group consisting of metallic tin, tin oxides and mixtures thereof.
In a preferred embodiment of the solid absorbent according to the invention the active absorption component is supported on a carrier material.
The carrier material for use in the inventive absorbent may be any known refractory materials which are stable at elevated temperatures. Suitable carrier materials are clays, silicates, alumina and titania.
Preferably, the carrier material is alumina or titania.
As tin is the active sulphide-capturing material the absorbent contains preferably at least 5% by weight of tin, tin oxides, or mixtures thereof.
In addition to the absorption component the solid absorbent according to the invention may further comprise a stabilizing component selected from group consisting of the oxides of copper, nickel, cobalt and iron.
The inclusion of the second component stabilizes the absorbent by forming an alloy with tin and thereby increasing the melting point of tin, which is formed during the purification of highly reducing gas mixtures.
A very dramatic raise of the melting point is seen in tin-iron alloys containing from 0.01 atom % to 20 atom % iron.
Accordingly, a particular preferred embodiment of the solid absorbent according to the invention comprises tin oxide as the active absorption component and from 0.01 atom % to 20 atom % of the active absorption component-iron or other stabilizing component.
According to the invention, the solid absorbent as mentioned hereinbefore may be prepared by any convenient method.
The absorbent is advantageously prepared by impregnating the carrier with an impregnation solution comprising the desired metal compounds and transforming the absorbent in a subsequent calcination step to its active form. Suitable tin salts for use in the impregnation solution are tin hydroxide and tin halogenides.
For the preparation of stabilized absorbents according to a preferred embodiment of the invention the impregnation solution further comprises salts of copper, nickel, cobolt or iron. Useful copper, nickel, cobalt and iron salts are any salts which by heating in a reducing atmosphere can be converted to their metallic form.
In a particularly preferred method the absorbent is prepared by pulverizing and intimately mixing particles of desired metal compounds and carrier material, succeeded by calcination to bring the absorbent in its active form for absorption of gaseous sulphides.
The activated absorbent may be used in any convenient shape, such es extrudates, tablets, pellets, granules, pills or powder.
In accordance with the inventive process the solid absorbent may be used in fixed bed manner in one or more reactors which operate alternatingly between absorption and regeneration phases. It may also be used in a fluidized bed and regenerated in a separator reactor.
During the absorption phase, sulphide-containing gases are passed through the reactor at a temperature between 200.degree. C. and 600.degree. C., as usually provided by the hot gases to be purified. Gaseous sulphides are thereby absorbed by tin and tin oxide according to one or more of the following reactions: EQU Sn+H.sub.2 S.rarw..fwdarw.SnS+H.sub.2 .DELTA.H.sub.0 =-19 kcal/mole(1) EQU SnO.sub.2 +H.sub.2 +H.sub.2 S.rarw..fwdarw.SnS+2H.sub.2 O; .DELTA.H.sub.0 =4 kcal/mole (2) EQU SnO.sub.2 +H.sub.2 +RSH.rarw..fwdarw.SnS+H.sub.2 O+ROH (3) EQU SnO.sub.2 +H.sub.2 +COS.rarw..fwdarw.SnS+CO.sub.2 +H.sub.2 O .DELTA.H.sub.0 =4 kcal/mole (4)
Regeneration of spent absorbent is predominantly effected by the reversed and substantially thermoneutral reaction (2) by passing a stream of hot steam through the reactor during the regeneration phase.
Since the regeneration thereby proceeds by nearly thermoneutral reactions the temperature during the regeneration phase may vary within wide ranges.
The regeneration phase is advantageously carried out at temperatures between 300.degree. C. and 500.degree. C. though lower temperatures are permissible without any risk of sulphate formation.